Originally Posted by orbitmic

If your daughter is very young (I believe 12 is the maximum limit), then BA will commit to seating you together one way or another. If she is older than that, you cannot be absolutely certain and indeed, there is always a small risk that no seats together are left, but depending on which cabin you are flying, this is not very likely: only a minority of people have status, and most people without status do not pay to pre-assign seats. What you could do is check the seat map 'as though' you were going to pay to see if many seats together are left. If at any point you see that there are only 2-3 pairs of seats left in your cabin, then you could make the decision to pay to be sure. If on the other hand there is plenty, then as c-w-s mentions, theoretical seating should ensure that you are not split, and checking in as soon as check in opens will further guarantee you the best choice of seats.

There are no way to avoid the payment if you really want to pre-assign, have no Oneworld status, are not flying on a full fare ticket, and did not book a specific corporate rate some of which include free pre-assignment as a perk.

Originally Posted by Filthy Monkey

Incidentally, for future reference, if you book through certain travel agents (Propeller Travel, for example), you get free seat assignment with BA. Worth the booking fee for even a single long haul sector.